Sensitivity to temperature perturbations of the ageing states in a re-entrant ferromagnet

Dynamic magnetic properties and ageing phenomena of the re-entrant ferromagnet (Fe_0.20Ni_0.80)₇₅P₁₆B₆Al₃ are investigated by time dependent zero field cooled magnetic relaxation, m(t), measurements. The influence of a temperature cycling (perturbation), ,(prior the field application) on the relaxation rate is investigated both in the low temperature re-entrant spin glass “phase” and in the ferromagnetic phase. In the ferromagnetic phase the influence of a positive and a negative temperature cycle (of equal magnitude) on the response is almost the same (symmetric response). The result at lower temperatures, in the RSG “phase” is asymmetric, with a strongly affected response for positive, and hardly no influence on the response for negative temperature cycles. The behaviour at low temperatures is similar to what is observed in ordinary spin glasses. Received 20 August 1998     

Frequency dependent magnetoconductivity and conductivity study in Ni-dispersed silica nano-composite produced by sol-gel technique

The low frequency (20 Hz to 1 MHz) ac conductivity and magnetoconductivity behaviour of ceramic nanocomposite (Ni-SiO₂) at low temperature down to 77 K are reported. The frequency dependent conductivity followed the power law, σ(ω) ∝ ω ^s . The fractional exponent s is a function of temperature and was found to increase with increasing temperature. This type of variation may be attributed to small polaron hopping. A peak present in the loss tangent indicates the presence of a Debye relaxation process. The magnetoconductivity of the samples is positive, which strongly depends on frequency. A firm theoretical explanation of frequency dependent magnetoconductivity is still lacking.     

Magnetic phase diagram of anisotropic layered superconductors via magnetization measurements forH‖c in Bi2212

The Bi₂Sr₂CaCu₂O₈ system is viewed as an archetypal of superconductors modelled as Josephson coupled CuO₂ bilayers. The isothermal and temperature dependent DC and AC magnetization measurements forH‖c in a single crystal of Bi2212 have been performed. Qualitative changes are observed to occur over a narrow range of temperature values before reaching the superconducting-normal transition. The observed behaviour can be ascribed to the rapid variation in the strength of the coupling between the superconducting CuO₂ planes (i.e., bilayers in the case of Bi2212). Strongly coupled planes behave like a 3D superconductor, whereas weakly coupled planes have a two component response attributable to 2D planes and interplanar couplings. We believe that this paper is a plethora of new findings. Our observations imply that resistivity across the planes becomes zero earlier than that within the planes. A new line (designated asH _2D(T)) above which the change in the electromagnetic response is dominated by quasi 2D-planes has been determined for the first time. This paper also contains the first observation of Differential Diamagnetic Effect (DDE) in the In-phase AC susceptibility data which signals the onset (atT _2D(H)) of dominance of response from 2D-planes. In addition to a host of interesting thermomagnetic history effects which are a consequence of interplay between the diamagnetic responses from the two components, a comparison of irreversibility lines (of the 3D state) determined by different methods on the same specimen of a HTSC is also being presented for the first time. We have come across Paramagnetic Meissner Effect (PME), first recognized in ceramic samples of Bi2212, in the temperature region of dimensional crossover in our single crystal sample, whichinter-alia confirms our labelling of the two component behaviour. A schematic phase diagram summarizing the various transformations that can occur nearT _c in the electromagnetic response of an anisotropic layered system has been drawn.     

Electrical properties of nickel-doped arsenic trisulphide

Results of temperature and frequency dependent a.c. conductivity of pure and nickel-doped a-As₂S₃ are reported. The a.c. conductivity of pure As₂S₃ obeys a well-known relationship: σ_ac ∝ω ^s. Frequency exponents is found to decrease with increasing temperature. Correlated barrier hopping (CBH) model successfully explains the entire behaviour of a.c. conductivity with respect to temperature and frequency for pure As₂S₃. But a different behaviour of a.c. conductivity has been observed for the nickel doped As₂S₃. At higher temperatures, distinct peaks have been observed in the plots of temperature dependence of a.c. conductivity. The frequency dependent behaviour of a.c. conductivity (σ_ac ∝ω ^s) for nickeldoped As₂S₃ is similar to pure As₂S₃ at lower temperatures. But at higher temperatures, ln σ_ac vs lnf curves have been found to deviate from linearity. Such a behaviour has been explained by assuming that nickel doping gives rise to some neutral defect states (D ^0′) in the band gap. Single polaron hopping is expected to occur between theseD ^0‘ andD ⁺ states. Furthermore, allD ⁺,D ^0′ pairs are assumed to be equivalent, having a fixed relaxation time at a given temperature. The contribution of this relaxation to a.c. conductivity is found to be responsible for the observed peak in the plots of temperature dependence of a.c. conductivity for nickel-doped As₂S₃. The entire behaviour of a.c. conductivity with respect to temperature and frequency has been explained by using CBH and “simple pair” models. Theoretical results obtained by using these models, have been found to be in agreement with the experimental results.     

Low temperature1H NMR study of electronic structure parameters (T 1e T) in zirconium dihydride

Measurements of the proton-spin-lattice relaxation times (T ₁) are reported for zirconium dihydrides: ZrH_1.87, ZrH_1.93 and ZrH_2.00 at nominal frequency of 41 MHz, with emphasis on the low temperature (4.2–25 K) behaviour of theT ₁. The dominant contributions are originated from the hyperfine interactions of the protons with the conduction electrons and the paramagnetic impurities. The (T _1e T)^–1/2 values determined from the low temperature range (4.2–25 K) agree with those obtained in the extended range (4.2–300 K) as well as with the values reported by the other researches. The paramagnetic impurity contribution is found to be small and teperature independent.     

Is the configurational entropic model able to predict the final equilibrium state reached by Se glasses after very long ageing durations?

This work compares the equilibrium states reached by glassy selenium (g-Se) after several ageing durations at temperatures lower than the glass transition temperature T _g, with the forecasts given by the configurational entropic model. The comparison is carried out through experimental data collected both on glassy samples after short-term ageing and on glassy samples older than 20?years, kept at room temperature. It is shown that the configurational entropic model is not able to describe the behaviour of g-Se, both at short- and long-ageing terms with the same set of fitting parameters. For short-term ageing, the forecasts given by the entropic model are in good agreement with the experimental data; the hypothesis that the heat capacities corresponding to the equilibrium state and to the state extrapolated from the liquid differ by an amount δ is therefore properly justified. For long-term ageing, the amount δ vanishes and the assumption does not hold any longer. Indeed, experimental results support the idea that at least two separate mechanisms of relaxation coexist in the physical ageing of g-Se, one with very fast kinetics and the other with much slower kinetics.     

Electrochemical oxidation of methanol on Pt nanoparticles composited MnO2 nanowire arrayed electrode

By use of the membrane-template synthesis route, MnO₂ nanowire arrayed electrodes are successfully synthesized by means of the anodic deposition technique. The Pt nanoparticles composited MnO₂ nanowire arrayed electrodes (PME) are obtained through depositing Pt on MnO₂ nanowire arrayed electrode by cathode deposition technique. For comparison of electrochemical performance, Pt nanowire arrayed electrodes which have the same amount of Pt with PME are also prepared. The electro-oxidation of methanol on PME and Pt nanowire arrayed electrodes is investigated at room temperature by cyclic voltammetry, which show that about 110 mV decreased overpotential and 2.1-fold enhanced votammetric current are achieved on PME. The chronoamperometry result demonstrates that the resistance to carbon monoxide for PME is improved.     

Electrical conductivity and structural behaviour of Al65Cu20Ru15 quasicrystalline alloy

The low temperature conductivity σ (T) and the magnetoresistance (up to 8 T) of a phason disorder free pure Al₆₅Cu₂₀Ru₁₅ quasicrystalline alloy, prepared by melt casting, has been investigated in the temperature range from 1.3 K to 200 K. In the temperature range 1.3 to 20 K the conductivity exhibits a -variation, whereas for the range 20 to 200 K a linearT-dependence has been observed. The analysis of magnetoresistance data reveals the presence of weak localization and electron-electron interaction effects. The positive nature of the magnetoresistance indicates the presence of spin orbit scattering. The σ∞ behaviour prevailing over such a wide temperature range has been observed for the first time for Al₆₅Cu₂₀Ru₁₅. A quantitative analysis of the -variation in terms of electron-electron interaction yields a very low density of states at the Fermi level.     

Low temperature electrical conductivity and magnetoconductivity of Fe39Ni39Mo4Si6B12 and Co58Ni10Fe5Si11B16 metallic glass alloys

A detailed study on the weak localization phenomenon vis-a-vis electron-electron interaction effects in magnetic metallic glasses has been carried out. We measured the electrical conductivity and magnetoconductivity within the temperature range 1.8≤T≤300K. A maximum on the conductivity versus temperature curve exists atT=T _m. The conductivity was observed to follow aT ^1/2 law forT<T _m andT ² law forT>T _m. Magnetoconductivity data of these alloys indicate the prominence of electron-electron interaction at low temperatures. The authors have determined the inelastic scattering field and spin-orbit scattering field from the magnetoconductivity data. The inelastic scattering field obeys aT ^p law (p=2) at low temperatures.     

Transport properties of Ti3Ir compound at low temperature

Electrical resistivity and magnetoresistivity of Ti₃Ir compound have been measured in the temperature range 2.0 K ≤T ≤ 300 K in absence as well as in presence of magnetic field upto 7.7 T. The low temperature resistivity shows aT ² behaviour whereas the high temperature resistivity shows a linear behaviour. The magnetoresistivity is positive and cannot be explained by simple s-d scattering model. The enhancement of the coefficient A of theT ² term and the deviation from the quadratic field dependence of the resistivity may be due to the anisotropy in the compound. This work has been performed under the grant by the Department of Science and Technology, Govt. of India.     

Superconductivity induced phonon anomalies in the Raman spectra of Zn and Ni doped YBa2Cu3O7

Here we present Raman spectra of YBa₂(Cu_1–x Zn _x )₃O₇ and YBa₂(Cu_1–x Ni _x )₃O₇ as a function of temperature and Zn or Ni content. The temperature dependence of two modes at 340 and 440 cm^–1 is analyzed. Similarly to the infrared measurements it is found that Zn substantially suppresses the superconductivity induced phonon softening whereas, Ni does not affect much that effect. Moreover, the superconductivity induced phonon stiffening of the 440 cm^–1 mode completely disappeared with the Zn doping. We find this behaviour might support the model where Zn acts effectively as a magnetic pair breaker.     

Jahn-Teller distortion and magnetoresistance in electron doped Sr1-xCexMnO3 (x = 0.1, 0.2, 0.3 and 0.4)

Neutron and electron diffraction, electrical transport and magnetic measurements have been carried out on a newly synthesized electron doped Sr_1-xCe _x MnO₃ (x = 0.1, 0.2, 0.3 and 0.4) system. For x=0.1, while cooling, it undergoes a first-order metal-insulator transition at 315 K which is associated with a structural transition from cubic (Pm3m) to tetragonal (I4/mcm) due to Jahn-Teller ordering () which stabilizes a chain like (C-type) antiferromagnetic ground state with . The antiferromagnetic insulator state is insensitive to an applied magnetic field of 7 T. With increase of x, while the nuclear structure at room temperature for x=0.2 and 0.3 remains tetragonal, for x=0.4 it becomes orthorhombic (Imma) where the doping electrons seem to occupy mainly the d _x2-y2 symmetry. Further, the JT distortion and the antiferromagnetic interactions decrease with doping and a small negative magnetoresistance appears for . Magnetic measurements show that the dilution of antiferromagnetic interaction results into a spin glass like behaviour at low temperature for the samples with x=0.3 and 0.4. This behaviour is in contrast with the CMR properties of calcium based electron doped systems and hole doped manganites. The stability of C-type antiferromagnetic ordering in the electron doped system with large A-site cationic size may be responsible for the absence of double exchange ferromagnetism and CMR effect. Received 10 September 1999     

Electrical properties of a-antimony selenide

This paper reports conduction mechanism in a-Sb₂Se₃ over a wide range of temperature (238 to 338 K) and frequency (5 Hz to 100 kHz). The d.c. conductivity measured as a function of temperature shows semiconducting behaviour with activation energy ΔE=0.42 eV. Thermally induced changes in the electrical and dielectric properties of a-Sb₂Se₃ have been examined. The a.c. conductivity in the material has been explained using modified CBH model. The band conduction and single polaron hopping is dominant above room temperature. However, in the lower temperature range the bipolaron hopping dominates.     

Slow relaxation experiments in disordered charge and spin density waves: collective dynamics of randomly distributed solitons

We show that the dynamics of disordered charge density waves (CDWs) and spin density waves (SDWs) is a collective phenomenon. The very low temperature specific heat relaxation experiments are characterized by: (i) “interrupted” ageing (meaning that there is a maximal relaxation time); and (ii) a broad power-law spectrum of relaxation times which is the signature of a collective phenomenon. We propose a random energy model that can reproduce these two observations and from which it is possible to obtain an estimate of the glass cross-over temperature (typically T _g≃ 100-200 mK). The broad relaxation time spectrum can also be obtained from the solutions of two microscopic models involving randomly distributed solitons. The collective behavior is similar to domain growth dynamics in the presence of disorder and can be described by the dynamical renormalization group that was proposed recently for the one dimensional random field Ising model [D.S. Fisher, P. Le Doussal, C. Monthus, Phys. Rev. Lett. 80, 3539 (1998)]. The typical relaxation time scales like ∼τexp(T _g/T). The glass cross-over temperature T_g related to correlations among solitons is equal to the average energy barrier and scales like T _g∼ 2xξΔ. x is the concentration of defects, ξ the correlation length of the CDW or SDW and Δ the charge or spin gap. Received 12 December 2001     

Infrared diode laser absorption features of N2O and CO2 in a laval nozzle

Design and operation of a pulsed Laval nozzle and the characterization of molecular flow through such a nozzle using IR tunable diode laser (TDL) is the central theme of this work. The results here deal with He diluted N₂O and CO₂ gaseous systems. Boltzmann type plots of the spectral intensity data of both N₂O and CO₂ show non-linear behaviour. We have attempted to understand this non-linear behaviour of Boltzmann plots in terms of (1) instability in the jet and (2) a two-temperature model for the flowing gas, a cold central core and a hot boundary layer close to the nozzle walls. The model based on jet instability represents the data somewhat poorer than the two-temperature model. The parameters derived from fitting our experimental data to the former model could be used to calculate the thermodynamic parameters only through further approximations. Measured absorption line profile of the P(15) line of the v ₂ band of N₂O as a function of axial distance from the nozzle exit gradually shifts from a Lorentzian to a Gaussian type. Velocity distribution of N₂O molecules in a Laval nozzle is determined by differentiating the absorption line profile of the P(15) line (v ₀=576.235 cm^–1) of the v ₂ band of N₂O. Translational temperature of N₂O molecules is determined from the observed spectral profiles.     

非晶态聚合物主转变弛豫峰与玻璃化转变过程的相关性研究

从弛豫理论出发,在不同的时效时间和不同降温速率下,测量了非晶聚合物PVC玻璃化转变过程中与时间参量密切相关的α峰峰温(T_α)的响应关系,得出以下结论:1)玻璃化转变温度(T_g)一般是指非晶态聚合物热平衡态与非平衡态的温度转变点(T_t),是温度变化造成的热弛豫过程的直接结果,而α内耗峰是在动态力学测量过程中得到的聚合物玻璃态与高弹态(橡胶态)的分界点处的力学弛豫过程,表现为动态模量锐变过程,是非晶态聚合物热平衡态下的特征内耗峰.两者都与非晶态     

Positron annihilation at defects in sintered high-T c perovskite superconductors

We report positron lifetime measurements in sintered superconducting YBa₂Cu₃O_7–x and GdBa₂Cu₃O_7–x oxides. It is shown that the thermal behaviour of the positron lifetime spectra strongly depends on the preparation of the ceramics. A lifetime of 190±3 ps is attributed to oxygen deficient regions. Two lifetimes of 251±7 ps and 225 ±5 ps are attributed to a cation vacancy presenting a temperature dependent atomic arrangement. The lifetime transition (251225ps) occurs during decreases in temperature across the resistivity superconducting transition. This lifetime change indicates that the volume of the cation vacancy decreases in the superconducting state.     

Growth and melting behaviour of thin in films on Ge(100)

Growth and melting behaviour of thin indium films on Ge(100) have been investigated by Auger-electron spectroscopy (AES), atomic force microscopy (AFM) and perturbed angular correlation (PAC) spectroscopy, respectively. At room temperature inidium is found to grow in three-dimensional islands even at submonolayer coverages. A very rough film surface is observed for thicknesses up to 230 ML. The melting behaviour of such films has been studied by PAC. A reduction of the melting temperature T _m as well as a strong supercooling of the films is observed. The electric field gradient for ¹¹¹In(¹¹¹Cd) in the indium islands is determined as a function of temperature and is used to monitor the local crystalline order of the films up to temperatures just below the melting point.     

Quantitative evaluation of substrate temperature dependence of Ge incorporation in GaAs during molecular beam epitaxy

Combined Hall effect and low-temperature photoluminescence measurements have been used to perform a thorough evaluation of the growth temperature dependence of Ge incorporation in GaAs during molecular beam epitaxy (MBE) over the entire substrate temperature range (400°≦T _s ≦600[°C]) practicable forn-type layer growth. Using a constant As₄ to Ga flux ratio of two, growth below 500°C yieldsn-GaAs: Ge films having electrical and optical properties rapidly deteriorating with decreasingT _s . Growth at 500° ≦T _s ≦600[°C] produces high-qualityn-GaAs: Ge films (N _D /N _A ≈4) with C as well as Ge residual acceptors competing on the available As sites. The amount of Ge atoms on As sites [Ge_As] increases with substrate temperature, whereas simultaneously the amount of C atoms on As sites [C_As] decreases thus leading to the well-establishednonlinear behaviour of the (N _A /N _D vs. 1/T _s plot. Counting the incorporated Ge impurities separately, however, yields alinear behaviour of the ([Ge_As]/[Ge_Ga]) vs. 1/T _s plot which has exactly the same slope as the (P _{As 2}/P _Ga) vs. 1/T _s plot derived from vapour pressure data of As₂ and Ga over solid GaAs surfaces. The important result is, therefore, that the incorporation behaviour of Ge in GaAs during molecular beam epitaxy is directly correlated with theevaporation behaviour of the growing GaAs surface.     

Enthalpy Relaxation Study of Poly(Vinyl Chloride) Films Based on the Time-Temperature Superposition Principle

Abstract

In the enthalpy relaxation of poly(vinyl chloride), a decrease in enthalpy upon the isothermal ageing was measured using the differential scanning calorimetry method as a function of ageing time (t_A) and ageing temperature. The range of the ageing temperature was from 56?°C (T_g ? 25?°C) to 72?°C (T_g ? 9?°C) where T_g denotes the glass transition temperature. The limiting value of the decrease in enthalpy was determined by applying a stretched exponential function to the measured enthalpy data. The relaxation function (?) was derived from the measured enthalpy and the construction of a master curve was tried by shifting the ? ? t_A curves of the respective ageing temperatures horizontally. Although there was no agreement between the shift factors (a_T) and the relaxation times of the ? ? t_A curves, the superposition was successfully constructed and the a_T values obtained for the poly(vinyl chloride) sample were found to be comparable to those reported for viscoelastic experiments over a broad temperature range above and below T_g carried out for different polymers. The origin of the decrease in enthalpy was briefly discussed in terms of the chain dynamics in the isothermal condition.